Flexographic printing is a method of direct rotary printing that uses a resilient relief image in a plate of rubber or photopolymer to print articles such as cartons, bags, labels or books. Flexographic printing has found particular application in packaging, where it has displaced rotogravure and offset lithography printing techniques in many cases. While the quality of articles printed using flexographic plates has improved significantly as the technology has matured, physical limitations related to the process of creating a relief image in a plate remain.
In particular, it is very difficult to print small graphic elements such as fine dots, lines and even text using flexographic printing plates. In the lightest areas of the image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represent the density of the image. In both the aforementioned cases, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small dots on a flexographic plate is very difficult due to the nature of the plate making process. Digital flexographic plate precursors have an integral UV-opaque mask layer coated over the photopolymer. In a pre-imaging (or post-imaging) step the floor of the plate is set by area exposure to UV light from the back of the plate. This exposure hardens the photopolymer to the relief depth required for optimal printing. This step is followed by selective ablation of the mask layer with an imagewise addressable high power laser to form an image mask that is opaque to ultraviolet (UV) light in non-ablated areas. Flood exposure to image-forming UV radiation and chemical processing follow wherein the areas not exposed to UV are removed in a processing apparatus using solvents or by a heating and wicking process. The combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not “held” or formed on plate and/or on press). Alternatively, if the dot survives processing they are susceptible to damage on press. For example small dots often fold over and/or partially break off during printing causing either excess ink or no ink to be transferred.
Conventional or non-digital flexographic platemaking follows a similar process except that the integral mask is replaced by a separate film mask or phototool that is imaged separately and placed in contact with the photopolymer plate precursor under a vacuum frame for the image-forming UV exposure.
In printing, it is well known that there is a limit to the minimum size of halftone dot that may be reliably represented on a plate and subsequently printed on press. The actual minimum size will vary with a variety of factors including plate type, ink, imaging device characteristics etc. This creates a problem in the highlight areas when using conventional AM screening since once the minimum dot size is reached, further size reductions will generally have unpredictable results. If, for example, the minimum size dot that can be printed is a 50×50 μm square dot, corresponding to a 5% tone at 114 lines per inch screen frequency, then it becomes very difficult to faithfully reproduce tones between 0% and 5%. A common workaround is to increase the highlight values in the original file to ensure that after imaging and processing, that all the tonal values in the file are reproduced as printing dots and are properly formed on the plate. However, the downside to this practice is the additional dot gain in the highlights, which causes a noticeable transition between inked and non-inked areas. Another well-known practical way of improving highlights is through the use of “Respi” or “double dot” screening. One such technique is shown in FIG'S. 1-A to 1-C. A screening grid 10 for conventional AM screen is shown in a simplified schematic format. The screening grid comprises a plurality of halftone cells 12. A halftone cell is an area wherein an AM halftone dot is grown from a low density, where only a small dot is placed at the centre of the cell, to a high density or solid where the cell is completely filled. In FIG. 1-A, dots 14 are placed only in every second halftone cell. Dots 14 have size corresponding to the minimum reliably reproducible dot. As the density of the screen is increased, dots 14 are increased in size as shown in FIG. 1-B at 16. At some point in increasing the density, the previously empty halftone cells are populated with minimum size dots 18 as shown in FIG. 1-C. The dots 16 may be held at a fixed size with increasing screen density while allowing dots 18 to grow. When all dots are the same size as dots 16, conventional AM screening takes over.
The problem with this type of screening technique when applied to flexographic printing is that the size of dot that may be printed in isolation is actually quite large, typically 40-50 μm in diameter. Even when using this technique, the highlights are difficult to reproduce without having a grainy appearance (which occurs when dots are spaced far apart to represent a very low density) and the printed dot may also suffer an undesirable dot gain.
There remains a need to improve the representation of small dots in flexographic printing processes.